Spring steel wires containing 0.6-0.8 mass % of C, 0.15-0.35 mass % of Si, and 0.3-0.9 mass % of Mn are known in the art. Such a steel wire is manufactured by being processed through steps of rolling→patenting (heating for γ-phase transition→isothermal transformation)→wire drawing→(coiling)→strain relief annealing (for example, at 300±30° C.).
However, it is a well-known fact that such a type of steel wire obtained by drawing a pearlite steel (generally called a piano wire or hard drawn steel wire: hereinafter shall be generically referred to as a piano wire) has a relatively low heat resistance.
Therefore, in high temperature environments where a permanent set resistance is required, quenched and tempered steel wires such as heat-resistant piano wires having a high Si content and oil tempered wires of Si—Cr steel (hereinafter shall be referred to as OT wire) have been used. Working environments requiring a heat resistance include a case of galvanizing a steel wire, for example, and it is customary to add Si to the steel in order to prevent or retard a decrease in strength in the course of the galvanization process.
In addition, it has been proposed that a steel wire having a high strength and toughness can be obtained by finely dividing cementite into microcrystals of a nano-order size. (Japanese Provisional Publication NO. 120407/96.)
However, the aforementioned prior arts have had a number of problems as follows:                (1) While important properties for steel wires include: a) high tensile strength, b) high toughness, and c) high fatigue strength, a high tensile strength is not necessarily compatible with a high fatigue strength in those steel wires to be processed through drawing. Generally, the tensile strength of a steel wire increases with its working ratio of drawing (reduction ratio). In addition, a fatigue strength cannot be increased without a comparatively high tensile strength. However, increasing the working ratio will result in increased micro defects of the material through plastic working, and such micro defects, when concentrated, will act as origins of earlier occurring fatigue fractures.        (2) A heat-resistant piano wire generally has a high Cr content and takes a longer time for its heat treatment (patenting), resulting in a lower productivity.        (3) The use of a heat-resistant piano wire as a steel wire to be galvanized or otherwise exposed to heat (at about 450° C. for about 30 seconds) is intended to limit or retard a decrease in strength, but not to provide a thermal permanent set resistance at about 200° C. or so. It is known in a parallel wire and the like steel wires that heat resistance is improved by increasing the Si content. In this respect, however, the purpose of using steel wires having a good heat resistance varies with their specific uses, the heat resistance for the case of parallel wire fundamentally aims at limiting the change in tensile strength of the wire small when subjected to galvanization. On the other hand, in the case of automobile engine valve springs exposed to intense heat in operation or automobile torsion bars heated to at about 200° C. when car bodies are bake-finished, important considerations include keeping the permanent set in the temperature range of about 100-200° C. small and at the same time providing desired fatigue properties. Thus, simply applying a chemical composition of such a parallel wire to a spring wire cannot bring forth satisfactory properties sufficient for a spring material. That is to say, while the Si addition in a parallel wire is reportedly said to be effective in improving its fatigue properties, this is mere a story of fatigue under repeated tension, which differs essentially from the fatigue properties required for a spring material. A decrease in surface hardness greatly affects the fatigue properties in a spring steel wire having a high Si content, although its influence on the fatigue properties is small in a parallel wire.        (4) As for a heat-resistant piano wire, even the delayed fracture properties important for a spring are not usually taken into consideration. Steel wire may sometimes be subjected to cationic coating and the like processing for an anticorrosion purpose, and delayed fracture may be caused then if hydrogen gets into it the steel wire. Especially, in a spring steel wire, delayed fracture properties to torsion stress are important, but such delayed fracture properties has hardly been taken account of so far.        (5) OT wire is expensive. While a steel wire superior in both heat resistance and fatigue strength can be obtained by applying quenching and tempering in the final stage of the steel wire manufacture, such a quenching and tempering process adds to the cost.        
Accordingly, an object of the present invention is to provide a steel wire having a high heat resistance (particularly at around 200° C.) and a high fatigue strength that can be produced without applying a quenching and tempering process, namely, produced through a drawing process and a method of manufacturing such a steel wire.
Another object of the present invention is to provide a steel wire having superior delayed fracture properties in addition to the heat resistance.
A further object of the present invention is to provide a steel wire having superior fatigue properties that can be achieved by improving its material strength and at the same time by optimally minimizing the origins of fatigue fracture and a method of manufacturing such a steel wire.